1. Field of the Invention
The present invention relates generally to inflatable packers for use in oil and gas wells for providing annular seals between the outside of pipe and the surrounding surface of the borehole or casing, and more particularly to an inflatable packer having an expansible reinforcing sheath that is pressure and volume balanced with the well bore.
2. Description of the Prior Art
Inflatable packers of the type disclosed, for example, in U.S. Pat. No. 3,640,723 or U.S. Pat. No. 3,837,947, have been used for many years. Such packers include a tubular mandrel that is covered by inflatable sleeve secured to the mandrel by a pair of axially spaced apart end assemblies. Each end assembly includes a collar, which is adapted to be connected to the mandrel, and an annular head, which is connected at one end to the collar and at the other end to the sleeve. The sleeve is normally reinforced by an expansible reinforcing sheath, which comprises a plurality of overlapping ribs connected at each end to a head. A passage with valve means is provided in one of the collars for allowing the passage of fluid from inside the pipe string between the inflatable sleeve and the mandrel to inflate the sleeve into sealing contact with the well bore or casing. Such inflatable packers function to isolate the annulus above the packer from that below, and accordingly, need only be of a length long enough to form an effective seal.
More recently, there have been developed inflatable packers for use in well completion, which are adapted to be positioned adjacent the producing zone and inflated with cement. After the cement has set, the packer is perforated and the well is produced through the packer. Examples of such inflatable packers are disclosed, for example, in U.S. Pat. No. 3,918,552, U.S. Pat. No. Re. 30,711 and U.S. Pat. No. 3,909,034. Such inflatable packers tend to be relatively long, i.e., from ten feet to forty feet in length, in order to seal against both the producing formation, which is perforated, and the formations above and below the producing formation.
Since the completion type inflatable packers are of such length, the central portion of the inflatable sleeve is supported and, in effect, reinforced by the borehole. Accordingly, a reinforcing sheath is unnecessary in the central part of the inflatable sleeve. However, reinforcing is necessary adjacent the ends of the inflatable sleeve to prevent the inflatable sleeve from blowing out and/or extruding past the heads. Therefore, the inflatable sleeves of completion packers are normally reinforced only at the ends adjacent the heads.
Inflatable packers are intended for use in high pressure environments. Since the well bores into which the packers are run are filled with drilling fluid or the like, during running and prior to inflation, the packers may be subjected to extreme hydrostatic pressures. When the reinforcing sheaths are completely enclosed in an outer rubber cover and the rib cavity is not fluid filled, bottom hole pressure acts directly on the ribs forcing them together. The coefficient of friction between the ribs, even when lubricated, is such that the frictional forces during the inflation sequence may prevent them from sliding with respect to one another and thereby prevent their expanding. In the case of continuous rib inflatable packers, such rib friction may cause the packer not to inflate. In the case of completion type packers, such rib friction may likewise cause the packer not to inflate or may cause only the unsupported central portion of the inflatable sleeve to expand and extrude past the reinforcing sheath.
It has been attempted to eliminate failures due to reinforcing sheath friction by injecting a specific volume of fluid into the reinforcing rib cavity prior to running the packer into the well bore in the hope that bottom hole pressures would act upon the fluid surrounding the ribs rather than on the ribs themselves. However, the success of this procedure is highly technique sensitive. The amount of fluid injected into the rib cavity must be precisely measured. If too little fluid is injected, fluid may not completely fill the cavity and permeate between the ribs and the forces acting on all of the ribs will not be effectively reduced. If too much fluid is injected, the excess fluid may flow to one location in the rib cavity and cause a bulge that may prevent the packer from being run in to the hole.
It is therefore an object of the present invention to provide an inflatable packer that overcomes the shortcomings of the prior art by being constructed such that the reinforcing ribs will expand properly regardless of well pressure.